Liquid flow sensor fox x-ray tubes

ABSTRACT

A housing ( 30 ) surrounds at least a portion of an x-ray tube ( 1 ). A cooling system ( 32, 32 ′) supplies a cooling liquid through the housing. The cooling system includes a pump ( 40, 40 ′) and a flow sensor system ( 60, 60 ′) which measures a pressure difference across the pump. A processor ( 80, 80′, 82, 82 ′) determines a cooling fluid flow rate from the pressure difference. A controller ( 81, 81′, 82, 82′, 107 ) limits operation of the x-ray tube based on the cooling fluid flow rate and a measured temperature of the cooling fluid to prevent x-ray tube overheating while minimizing cooling time between x-ray tube operations.

CROSS REFERENCE TO RELATED CASES

Applicants claim the benefit of Provisional Application Ser. No.60/536,074, filed Jan. 13, 2004.

The present application relates to the x-ray tube arts. The inventionfinds particular application in monitoring the flow of a cooling liquidto an x-ray tube and will be described with particular referencethereto. It will be appreciated, however, that the invention findsapplication in a variety of fluid systems where it is desirable tomonitor fluid flow or thermal characteristics.

x-ray tubes typically include an evacuated envelope made of metal,ceramic, or glass which is supported within an x-ray tube housing. Theenvelope houses a cathode assembly and an anode assembly. The cathodeassembly includes a cathode filament through which a heating current ispassed. This current heats the filament sufficiently that a cloud ofelectrons is emitted, i.e. thermionic emission occurs. A high potential,on the order of 100-200 kV, is applied between the cathode assembly andthe anode assembly. The electron beam strikes the target with sufficientenergy that x-rays are generated, along with large amounts of heat.

An x-ray tube housing surrounding the tube defines a flow path for acoolant fluid, such as oil, to aid in cooling components housed withinthe envelope. In order to distribute the thermal loading created duringthe production of x-rays, a constant flow of cooling liquid ismaintained throughout x-ray generation. After circulating through thex-ray tube housing, the cooling liquid is passed through a heatexchanger. The optimum flow rate of cooling liquid depends on a numberof factors, including the x-ray tube power, its duty cycle, and theeffectiveness of the cooling system. In the event that the liquid flowrate drops below a minimum level, for example, due to pump malfunction,overheating of the x-ray tube components tends to occur, which isdetrimental to the lifetime of the tube.

Various systems have been developed to monitor liquid flow in an x-raytube cooling system. In one system, a flow switch is positioned in thepath of the fluid flow. As the liquid flows through the switch, theliquid displaces a magnet, which in turn actuates a hermetically sealedreed switch. A positive spring return deactivates the switch when theflow decreases. A flow indicator, such as a paddle wheel, is often usedtogether with the flow switch to provide a visual flow indicator. Theliquid passing the flow indicator spins the wheel, visually indicatingflow speed.

Because both the flow switch and flow indicator are installed in linewith the liquid flow, their presence inevitably creates flow resistancewhich reduces the liquid flow rate. This reduces the cooling capacity ofthe cooling system.

In an alternative system, a pressure switch is used to monitor theliquid flow indirectly. The pressure switch is usually installed at theoutlet of the pump used to circulate the cooling fluid. If the detectedpressure decreases below a preselected level, the pressure switchautomatically shuts down the x-ray tube. A sharp drop in pump pressureis often an indicator that the pump is losing power or failing.

In the case of the pressure switch, however, pump outlet pressure doesnot always accurately predict flow rates. For example where flow linesof the cooling system become partially obstructed or twisted, the pumppressure tends to increase as the pump works harder to maintain flowthrough the obstruction. As the pump starts to fail, the pressure“drops” to normal, but the flow, due to the obstruction, is belownormal. Thus, the pressure switch does not always protect the x-ray tubefrom overheating due to the loss in liquid flow.

The temperature of the cooling fluid within the x-ray tube housingdepends not only on the flow rate, but also on other factors, such asthe duty cycle power. An algorithm computes the maximum power which canbe used in a subsequent scanning operation, based on the duty cycle, thetube heat storage, and a predicted temperature in the cooling liquid.Over time, the accuracy of the algorithm computations decreases due toincreasing differences between the actual and the predicted temperaturesand cooling rates. To compensate for these inaccuracies, the x-ray tubeis often removed from service for an extended period during the day,such as an hour or more at mid day, to allow the x-ray tube to cool to aknow set point.

The present invention provides a new and improved method and apparatuswhich overcome the above-referenced problems and others.

In accordance with one aspect of the present invention, an assembly isproved. The assembly includes an x-ray tube. The x-ray tube includes anenvelope which defines an evacuated chamber in which x-rays aregenerated. A housing surrounds at least a portion of the envelope. Acooling system circulates a cooling liquid through the housing to removeheat from the x-ray tube. The cooling system includes a pump and a flowsensor system which is responsive to a pressure difference across thepump.

In accordance with another aspect of the invention, a method forcontrolling operation of an x-ray tube is provided. The method includescirculating a cooling fluid through a housing and over the x-ray tubewith a pump. Heat is removed from the cooling fluid which has circulatedthrough the housing. A flow rate of the cooling fluid is determined.This step includes determining a pressure difference across the pump ora function which correlates with the pressure difference and determiningthe flow rate from the pressure difference or function.

In accordance with another aspect of the invention, a system forremoving heat from an associated x-ray tube assembly is provided. Thesystem includes a fluid flow path which carries a cooling fluid to atleast a portion of the associated x-ray tube, and removes heattherefrom. A pump circulates the cooling fluid through the fluid flowpath. Means are provided for determining a pressure difference acrossthe pump. Means responsive to the determined pressure difference areprovided for controlling operation of the x-ray tube.

One advantage of at least one embodiment of the present invention isthat it enables flow rates in an x-ray tube cooling system to bedetermined.

Another advantage of at least one embodiment of the present invention isthat it enables flow rates to be determined without reducing the liquidflow.

Another advantage of at least one embodiment of the present invention isthat x-ray tube down time is reduced due to a more accurate predictionof x-ray tube power capabilities.

Another advantage resides in extending x-ray tube life.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 is a diagrammatic illustration of an x-ray tube and coolingsystem according to a first embodiment of the present invention;

FIG. 2 is a more detailed diagram of the x-ray tube and cooling systemof FIG. 1;

FIG. 3 is a schematic view of the pressure sensing system of FIG. 2;

FIG. 4 is an exemplary plot of liquid flow rate in gallon/minute (GPM)vs. the differential pressure across a pump in Bar;

FIG. 5 is an exemplary plot of the differential pressure across a pump(Bar) vs. transducer output in millivolts (mV);

FIG. 6 is an exemplary plot of liquid flow rate (GPM) vs. transduceroutput obtained from the plots of FIGS. 4 and 5;

FIG. 7 is a diagrammatic view of an x-ray tube and cooling systemaccording to a second embodiment of the present invention; and

FIG. 8 is a perspective view of a CT scanner incorporating an x-ray tubeand cooling system according to the present invention.

With reference to FIG. 1, a schematic view of a rotating anode x-raytube 1 of the type used in medical diagnostic systems, such as computedtomography (CT) scanners, for providing a beam of x-ray radiation isshown. The tube includes an anode assembly 10, which is rotatablymounted in an evacuated chamber 12, defined by an envelope or frame 14,typically formed from glass, ceramic, or metal. The x-ray tube anodeassembly 10 is mounted for rotation about an axis via a bearing assemblyshown generally at 16. A heated element cathode assembly 18 supplies andfocuses an electron beam A. The cathode is biased, relative to theanode, such that the electron beam is accelerated to the anode andstrikes a target area 20 of the anode. The beam striking the target areais converted in part to heat and in part to x-rays B, which are emittedfrom the x-ray tube through a window 22 in the envelope. The anode isrotated at high speed during operation of the tube. It is to beappreciated that the invention is also applicable to stationary anodex-ray tubes, rotating cathode tubes, and other electrode vacuum tubes.

A housing 30 filled with a heat transfer and electrically insulatingcooling fluid, such as a dielectric oil, surrounds the envelope 14. Thecooling fluid is directed to flow past the insert that includes thewindow 22, the bearing assembly 16, cathode assembly 18, and otherheat-dissipating components of the x-ray tube. The cooling fluid iscooled by a cooling system 32, which receives heated cooling liquid fromthe housing through an outlet line 34 and returns cooled cooling liquidvia a return line 36. The lines 34, 36 may be in the form of flexiblehoses, metal tubes, or the like.

In the illustrated embodiment, the housing 30 is shown as a unitarystructure defining an interior cooling space 38 which cools the entirex-ray tube 1. However, it will be appreciated that the housing mayinclude different regions, which are associated with different portionsof the x-ray tube, to allow separate or focused cooling of componentswhich are more prone to overheating. Indeed, the housing may constitutemultiple cooling housings, which may be interconnected by fluid lines,or separately connected with the cooling system. Additionally, it isalso contemplated that there may be more than one outlet and/or returnline to the housing.

With reference now to FIG. 2, the cooling system 32 includes a liquidpump 40, having an inlet 42, through which cooling fluid enters achamber 44 of the pump, and an outlet 46, through which cooling fluidleaves the pump chamber 44. A heat exchanger 48 removes heat from thecooling liquid prior to return of cooling liquid to the housing. In theillustrated cooling system 32, heated liquid flows along a fluid flowpath 33 via the outlet line 34 to the liquid pump, then by anintermediate fluid line 50 from the pump 40 to the heat exchanger 48,and finally returning to the housing via the return line 36. Within thehousing 30, the cooled cooling liquid circulates around the x-ray tube1, or components thereof, removing heat before exiting from the outletline 34. However, it will be appreciated that the positions of the pumpand the heat exchanger may be reversed such that the cooling liquid fromthe housing is cooled prior to reaching the pump.

A system 52 for detecting a pressure difference across the pump 40includes a non-obstructing flow sensor system 60, such as a differentialpressure transducer. The transducer 60 is responsive the pressuredifference across the pump and provides an electrical signalcorresponding thereto. Specifically, the pressure transducer 60 isconnected with a wall 62 of the inlet 42 by a first fluid line 64 andwith a wall 66 of the pump outlet 46 by a second fluid line 68. Thefluid lines 64 and 68 terminate at first and second diaphragms 70, 72 ofthe transducer, which respond to changes in the pressure in lines 64 and66 by exhibiting volumetric changes. The changes in the diaphragms aredetected by one or more volumetric detection sensors (not shown) withinthe pressure transducer 60 and converted to electrical voltages.

The transducer 60 does not obstruct the flow of liquid in the coolingsystem flow path 33, since no liquid flows through the transducer. Thisavoids reduction in the flow of liquid caused by the flow measuringequipment. Additionally, in the event of a blockage or kink in one ofthe cooling lines 34, 36, 50, which comprise the flow path 33, thereduced flow downstream of the pump 40 is recognized as an increase inpressure by the downstream diaphragm 72 with no increase or a decreaseon upstream diaphragm 70 and the transducer responds accordingly.

With reference now to FIG. 3, power for the transducer 60 is supplied bya power source 76, such as a DC power supply. The DC power supply isoptionally tapped from the main power source of the x-ray tube andrectified. Alternatively, a separate power source, such as a set ofbatteries is employed. The use of batteries tends to reduce the risk ofinterference of electrical signals from the electrical system of thex-ray tube and thus helps to increase the accuracy of the flowmeasurements.

With continued reference to FIG. 3, the detection system 52 furtherincludes a processing means 80, such as a microprocessor. Themicroprocessor 80 receives a signal output from the differentialpressure transducer. In one embodiment, the transducer 60, in responseto a pressure difference between the inlet 42 and the outlet 46, signalsan output voltage to the microprocessor 80. In an alternativeembodiment, the transducer 60 signals first and second voltagescorresponding to the input and output sensed volumetric changes. Themicroprocessor 80 then determines the differential voltage. In bothembodiments, the microprocessor 80 converts the signal(s) from thetransducer 60 to flow rate measurements, or a correlated function, inreal time.

While a transducer 60 is a preferred non-obstructing flow sensor systemit is also contemplated that the system 60 may alternatively includefirst and second independent flow sensors (not shown), upstream anddownstream of the pump, respectively. Each of the flow sensorsoptionally includes a diaphragm similar to diaphragms 70, 72 and anassociated volumetric sensor for detecting volumetric, pressure,fluxation, or other pressure indicating changes to the diaphragm. Thetwo flow sensors independently send signals to the processor 80, whichuses the signals to determine the differential pressure and or flowrate.

There is a relationship between the liquid flow rate in the coolingsystem 32 and the pressure difference across the pump 40 (headpressure), which is determined experimentally and then used to create acorrelation. A typical plot of liquid flow rate in gallons per minute(GPM) vs. the pressure difference across a pump 40 is illustrated inFIG. 4 (1 gallon=3.785 liters). There is also a relationship between thetransducer output voltage and the head pressure. A typical plot of headpressure vs. the transducer output is illustrated in FIG. 5. Theillustrated plot was obtained using an OMEGA PX26 differential pressuretransducer which uses a 10VDC power and produces a voltage signal thatis proportional to the differential pressure. By combining these twoplots (FIGS. 4 and 5), a correlation between liquid flow rate as afunction of transducer output is obtained, as illustrated in FIG. 6.Thus, the pressure difference detected by the transducer 60 can be usedto monitor the flow rate through the cooling system and hence throughthe housing 30.

With reference once more to FIG. 2, the microprocessor 80 is programmedto initiate a response if the detected flow rate (or electrical signalscorresponding thereto) falls below a predetermined safe level. Forexample, the microprocessor 80 also serves as a control means 81 whichsignals a power switch 82, when the flow rate falls below thepredetermined safe level. The power switch 82 responds by immediatelyshutting down power to the cathode 18 (or at least reducing the power tothe cathode).

Alternatively or additionally, the processing means 80 employs analgorithm or pre-programmed look-up table to determine the energy thatthe x-ray tube can sustain, without risking overheating, e.g., themaximum operating time at a selected power level. In one embodiment, inthe event that the determined flow rate suggests that the x-ray tube islikely to overheat if it is used without allowing a sufficient cool downtime, the control means 81 of microprocessor 80 provides a prompt to auser of the x-ray tube, e.g., via a video display screen 84, to indicatethat a cool down time should be allowed before the x-ray tube is usedfor further generation of x-rays. The processor 80 calculates a suitablecool down time and optionally overrides attempts to operate the x-raytube until the time period is over or the x-ray tube has cooled to amaximum allowable starting temperature.

In one embodiment, the processing means 80 is the microprocessorassociated with a control system for a radiographic device in which thex-ray tube is operated, such as a CT scanner.

While the transducer 60 is illustrated as being outside the pump 40, itis also contemplated that the transducer and optionally also theprocessing means 80 may be integral with the pump.

With reference now to FIG. 7, an alternative embodiment of a coolingsystem for an x-ray tube is shown. Similar elements of the coolingsystem are identified by a primed suffix (′) and new elements are givennew numbers. One or more temperature sensors, such as resistancethermometers, or the like, detect the temperature of the cooling liquid.In the illustrated embodiment, two temperature sensors 90, 92 measurethe temperature of the cooling liquid at or adjacent inlet and outlet94, 96, respectively, of the housing 30. For example, the sensors 90, 92may be positioned in the outlet and return lines 34′, 36′, respectively.It is also contemplated that the sensor or sensors 90, 92 couldadditionally or alternatively be positioned in contact with the coolingfluid within the housing 30.

The temperature sensors 90, 92 are connected with a processing means,such as a processor 80′. The sensors respond to temperature changes inthe cooling liquid and send detected temperatures or signalsrepresentative thereof to the processor 80′. The processor also receivessignals from the transducer 60′ in real time. The processor 80′ includesalgorithms, precalculated look-up tables, or other means for convertingthe signals from the temperature sensors and transducer into real timecooling fluid temperatures and cooling liquid flow rates. The processoralso includes a thermal algorithm or other means for computing aparameter of the x-ray tube, such as the x-ray tube heat storage in realtime and/or maximum energy (power-time) at which the x-ray tube canoperate without risking overheating, based on the computed flow andtemperatures and duty cycle power and time. This information is used tocontrol a device, such as a CT scanner, which makes use of the x-raytube 1.

It will be appreciated that in place of receiving inputs fromtemperature sensors, the processor 80 can use a conventional algorithmor other means to predict the cooling fluid temperature.

An exemplary CT scanner 100 is illustrated in FIG. 8. The CT scannerradiographically examines and generates diagnostic images of a subjectdisposed on a patient support 102. More specifically, a volume ofinterest of the subject on the patient support 102 is moved into anexamination region 104. An x-ray tube assembly 1 with an associatedcooling system 32′ is mounted on a rotating gantry 105 and projects oneor more beams of radiation through the examination region 104 to anx-ray detector 106.

A scan controller 107 controls the scanner 100 including the x-ray tube1 to perform a selected scan protocol, such as a single revolutionmultislice scan, a helical scan, a multiple revolution examination tomonitor physiological changes or evolution, such as a cardiac scan toimage selected cardiac phases, a contrast agent uptake scan, and thelike, a fluoroscopic exam, a pilot scan, and the like. The scanprotocols can have different durations, different x-ray tube dutycycles, and different tube operating powers.

The electrical signals from the detectors 106, along with information onthe angular position of the rotating gantry, are digitized byanalog-to-digital converters. The digital diagnostic data iscommunicated to a data memory 110. The data from the data memory 110 isreconstructed by a reconstruction processor 112. Volumetric imagerepresentations generated by the reconstruction processor are stored ina volumetric image memory 114. A video processor 116, which may be thesame as processor 80′, withdraws selective portions of the image memoryto create slice images, projection images, surface renderings, and thelike, and reformats them for display on a monitor 118 such as a video orLCD monitor.

During a scanning procedure, the processor 80′ receives temperature andpressure differential information from the temperature sensors 90, 92and pressure transducer 60′. The processor may also receive inputs suchas cycle power and number of slices to be examined in the next patientexamination process from a touch screen, key pad, or other input device120.

The processor 80′ employs a thermal algorithm or means to determine acooling condition of the x-ray tube housing 30 which corresponds to theheat stored in the x-ray tube in real time. The processor 80′ uses thecooling condition and the next scan parameters to predict whether thenext scanning procedure will cause the x-ray tube cooling fluid toexceed a maximum safe temperature or heat storage value and thuspotentially cause damage to the x-ray tube. This allows optimization ofthe time between scanning procedures, steps in a scanning procedure,patient ordering, and the like. The maximum safe temperature is based oninformation available about the performance of the particular type ofx-ray tube and includes a margin of error for ensuring safety of thex-ray tube.

A typical scanning procedure proceeds as follows:

1. The pump 40, 40′ pumps cooling fluid through the x-ray tube housing30.

2. The transducer 60, 60′ continuously or intermittently monitors thepressure difference of the pump and sends signals to processor.

3. The temperature sensors 90, 92 (where present) continuously orintermittently monitor cooling fluid temperature at the inlet and outlet94, 96 of the housing 30 and send signals to processor 80′.

4. An operator inputs selectable parameters of a scanning procedure,such as the number of slices through the processor input 120, such as akeyboard.

5. The processor 80, 80′ inputs appropriate selectable parameters andsignals from the temperature sensors and transducer 60, 60′ to analgorithm which determines the heat storage (or temperature) of thex-ray tube cooling fluid as a function of time.

6. The processor 80, 80′ and the scan controller 107 control theoperation of the scanning procedure to optimize time between scans whilemaintaining the heat storage of the x-ray tube below a predeterminedmaximum level. Alternatively, the processor shuts off power to the x-raytube until the heat storage of the x-ray tube drops to a preselectedlevel to allow the scanning procedure to proceed without exceeding thepredetermined maximum heat storage of the x-ray tube.

7. In the event that the processor detects that the maximum heat storage(or temperature) has been achieved, the processor 80, 80′ signals thepower switch 82′ or scan controller 107 to switch off power immediatelyto the x-ray tube.

The invention has been described with reference to the preferredembodiment. Modifications and alterations will occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. An assembly comprising: an x-ray tube; a housing which surrounds atleast a portion of the x-ray tube; a cooling system which circulates acoolant through the housing to remove heat from the x-ray tube, thecooling system including a pump and a flow sensor system capable ofmeasuring a pressure difference across the pump and determining acoolant flow rate from the measured pressure difference; and acontroller for controlling operation of the x-ray tube in response tothe sensed flow rate.
 2. The assembly of claim 1, wherein the flowsensor system includes a differential pressure transducer.
 3. Theassembly of claim 1, wherein the cooling system further includes: arecirculating fluid flow path including a first fluid line whichconnects the housing with an upstream end of the pump, and a secondfluid line which connects a downstream end of the pump with the housing,and wherein the measured pressure difference is a pressure differencebetween the first fluid line and the second fluid line.
 4. The assemblyof claim 1, wherein the measured pressure difference is a differencebetween a first pressure upstream of the pump and a second pressuredownstream of the pump.
 5. The assembly of claim 1, further including aprocessor capable of receiving a signal from the flow sensor system thatis correlated with the measured pressure difference and capable ofdetermining the flow rate from the signal.
 6. The assembly of claim 1,wherein the controller controls operation of the x-ray tube in the eventthat the determined flow rate is below a preselected minimum level. 7.The assembly of claim 1, wherein the controller is capable ofcontrolling at least one of: operating power of the x-ray tube;operating time of the x-ray tube; selection of a scan protocol; and thelength of a cooling period of the x-ray tube.
 8. The assembly of claim1, further including: a temperature sensor for sensing a temperature ofthe coolant circulating in at least one of the housing and the coolingsystem.
 9. The assembly of claim 8, further including: a processorcapable of receiving signals from the temperature sensor and the flowsensor system and determining from the received signals an indication ofthermal loading or remaining thermal capacity of the cooling system. 10.The assembly of claim 9, wherein the processor is capable of determiningthe length of a cooling period based on the determined indication, andbased on an x-ray tube power, operating time, and duty cycle of aplanned scan protocol, so as to ensure that the x-ray tube is capable ofperforming the planned protocol without overheating.
 11. The assembly ofclaim 1, wherein the x-ray tube is the x-ray tube of a CT scanner.
 12. ACT-scanner comprising: the assembly of claim 1; an x-ray detector; ascan processor; and a display.
 13. A method for controlling operation ofan x-ray tube, the method comprising: circulating a cooling fluidcoolant through a housing which surrounds at least a portion of thex-ray tube using a pump; removing heat from the coolant which hascirculated through the housing; determining a flow rate of the coolantby measuring a pressure difference across the pump or a function whichcorrelates with the pressure difference; and controlling operation ofthe x-ray tube in response to the determined flow rate.
 14. The methodof claim 13, wherein the step of controlling operation of the x-ray tubecomprises reducing power to the x-ray tube when the determined flow ratedrops below a predetermined minimum value.
 15. The method of claim 13,further including: detecting at least one temperature of the coolant.16. The method of claim 15, further including: determining a temperaturedifference between two temperatures of said at least one temperature.17. The method of claim 15, further including: determining a thermalloading condition of the x-ray tube from the detected temperature andthe determined flow rate.
 18. The method of claim 17, further including:in response to the determined thermal loading condition, controlling atleast one of: operating power of the x-ray tube; operating time of thex-ray tube; selection of a scan protocol; and, the length of a coolingtime of the x-ray tube.
 19. A system for removing heat from anassociated x-ray tube, the system comprising: a fluid flow path forcarrying a cooling fluid to at least a portion of the associated x-raytube and removing heat therefrom; a pump for circulating the coolingfluid through the fluid flow path; a sensor for determining a pressuredifference across the pump; and a controller responsive to thedetermined pressure difference for controlling operation of the x-raytube.
 20. The system of claim 19, further comprising a processor fordetermining cooling fluid flow rate from the determined pressuredifference.
 21. The system of claim 20, further including: a sensor fordetermining a temperature of the cooling fluid, wherein the controlleris responsive to the determined temperature.
 22. The system of claim 21,further including: a means for selecting a scan protocol; and a meansfor implementing a scan with the selected scan protocol; wherein thecontroller controls at least one of: operating power of the x-ray tube;operating time of the x-ray tube; selection of a scan protocol; and thelength of a cooling period of the x-ray tube.